Computable numbers

Alan Turing, regrettably far earlier than today
Alan Turing is one of my biggest fans, er..., that may have come out the wrong way around.

There are many accounts of his life and work, and in every one of them the genius clearly shines through.  It's not just that he worked brilliantly on things like computability and the development of the first electronic computers, but also that as early as the 1930's he was already starting to think in ways we're still developing today.

Then there is the (now) highly celebrated time at Bletchley Park during the war (although I feel it only fair to point out there there were many other brilliant and hard working people there as well...  there's a tendency to credit Turing with the whole circus.)

And then there is his tragic and wholly unnecessary death.  I don't usually rate government apologies issued decades after the event; I tend to shout "you weren't even born" at the radio...  however this one seems somehow appropriate.  The subsequent pardon is neither here nor there, obviously there was no crime to pardon, and the authorities of the day were obviously suffering a kind of mass delusion.  (Authorities do that a lot—the present day is no exception; please remember to hold them to account at every opportunity.)

Such is Turing's legacy that his name features in several bits of common computer science terminology still in common usage, and some of them come up in the poem below.  However the one that says the most doesn't feature.  This is is "Turing complete".  A Turing complete system can calculate anything which is calculable—given enough space and time.

2012 was officially "The Year of Alan Turing" but this poem dates from a couple of years before that.  I always felt I should do something to celebrate 2012 but in the event was far too busy working full time programming electronic computing apparatus.  So I guess I did celebrate, in my own way...

Computable numbers


Turing machine is how we remember
your name, a device which might—
given enough paper tape—
calculate anything,

except the mechanism cannot reach.
Not everything is computable, a point
you wanted made. Sometimes
there's no route from where you start
to the number you desire. 


Turing test is how we remember
your name, a sort of exam
where machines sit at consoles
and apply to join the human race.

Did you suspect you'd made a faith
when you invented this scrap
of applied philosophy? Some people need,
and passionately believe, that one day
the test will be passed.
They will look at a machine,
to see it looking back.

Did you imagine
your machines might be free—
as if paper tape could really be infinite—
and permit attachment
to whatever device
took their fancy:
dancing with lawnmowers by moonlight
or taking a chance
on an upright pillar-drill? 


Code-breaker Turing is how we remember
you—genius applied to a problem
of ideology, ice-cold salt water, and steel.
How did it begin?

Maybe in a smoke-filled room,
a serious man with a pipe explained:
All we (puff) need
is a new branch of mathematics,
(puff) a new kind of engineering,
a love for doing crosswords in German,
and total
(puff-puff) secrecy.

I like to think you hesitated
for only a second,
before rolling up your sleeves. 


Dear, dead Alan, if you had seen
all these future machines,
their imaginary rooms
where you can go to meet
even a pillar-drill. Perhaps
you could jot down suitable numbers.
Perhaps you wouldn't need
the apple.


  1. I love the poem. After a long day and half way through a Bloody Mary, that is about as specific as I'll get, but I love the poem.

  2. Thanks Margo! I'm half way down a glass of Merlot myself...